An implantable medical device may connect to one or more implantable conductors that are outside of the device. For example, an implantable cardiac rhythm management device (e.g., a pacemaker, a defibrillator, or a cardioverter) may connect to one or more leads implanted in or near the heart of a patient to monitor cardiac function and provide therapy for a patient who suffers from cardiac arrhythmia. For example, the implantable device may process signals received via implanted cardiac leads to track the type and timing of native cardiac signals. In the event cardiac events are not occurring at appropriate times or undesired cardiac events are detected, the implantable device may apply cardiac stimulation signals (e.g., pacing signals) to the heart via the implanted cardiac leads in an attempt to restore normal cardiac rhythm.
Under certain circumstances, however, unintentional pacing may occur when a patient with an implantable medical device is subjected to strong electromagnetic fields. For example, time-varying magnetic fields generated during magnetic resonance imaging (MRI) may induce currents in an implanted lead that may, in turn, stimulate (e.g., cause capture of) cardiac tissue. In some cases, this unintended pacing may cause cardiac fibrillation. Such MRI-induced currents may arise in different ways.
In some cases, pulsed magnetic gradients used during MRI scanning may induce voltage in an implanted cardiac lead connected to an implanted device. If such voltage appears across sufficiently low impedance, the resulting current flowing through the lead may cause stimulation of the heart.
In some cases, pulses of amplitude modulated radiofrequency (“RF”) energy from MRI scanning (e.g., with a carrier at 64 MHz or 128 MHz) may enter the implantable medical device, whereupon the pulses are rectified by internal circuitry of the implantable medical device. The rectified signal may then exit the implantable medical device as a lower frequency demodulated signal on the implanted lead. This lower frequency signal may then travel to the patient's heart via the implanted lead and potentially cause unintended stimulation.
In view of the above, a physician may elect to not prescribe MRI scanning for a patient who has an implanted medical device. Consequently, such a patient may receive suboptimal treatment. Accordingly, a need exists for implantable medical devices that are sufficiently immune to the influence of MRI magnetic fields and other electromagnetic interference (EMI). This would enable, for example, a patient who has an implanted MRI-compatible medical device to have no extra restrictions going under an MRI scan as compared to a patient who does not have such an implanted device.